Objets à références faibles
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Python gère les *références faibles* comme des objets de première
classe. Il existe deux types d'objets spécifiques qui implémentent
directement les références faibles. Le premier est un objet de
référence simple, et le second agit autant que possible comme un
mandataire vers l'objet original.

int PyWeakref_Check(PyObject *ob)

   Return true if *ob* is either a reference or proxy object.  This
   function always succeeds.

int PyWeakref_CheckRef(PyObject *ob)

   Return true if *ob* is a reference object.  This function always
   succeeds.

int PyWeakref_CheckProxy(PyObject *ob)

   Return true if *ob* is a proxy object.  This function always
   succeeds.

PyObject *PyWeakref_NewRef(PyObject *ob, PyObject *callback)
    *Valeur de retour : nouvelle référence.** Part of the Stable ABI.*

   Return a weak reference object for the object *ob*.  This will
   always return a new reference, but is not guaranteed to create a
   new object; an existing reference object may be returned.  The
   second parameter, *callback*, can be a callable object that
   receives notification when *ob* is garbage collected; it should
   accept a single parameter, which will be the weak reference object
   itself. *callback* may also be "None" or "NULL".  If *ob* is not a
   weakly referencable object, or if *callback* is not callable,
   "None", or "NULL", this will return "NULL" and raise "TypeError".

PyObject *PyWeakref_NewProxy(PyObject *ob, PyObject *callback)
    *Valeur de retour : nouvelle référence.** Part of the Stable ABI.*

   Return a weak reference proxy object for the object *ob*.  This
   will always return a new reference, but is not guaranteed to create
   a new object; an existing proxy object may be returned.  The second
   parameter, *callback*, can be a callable object that receives
   notification when *ob* is garbage collected; it should accept a
   single parameter, which will be the weak reference object itself.
   *callback* may also be "None" or "NULL".  If *ob* is not a weakly
   referencable object, or if *callback* is not callable, "None", or
   "NULL", this will return "NULL" and raise "TypeError".

PyObject *PyWeakref_GetObject(PyObject *ref)
    *Valeur de retour : référence empruntée.** Part of the Stable
   ABI.*

   Return the referenced object from a weak reference, *ref*.  If the
   referent is no longer live, returns "Py_None".

   Note:

     This function returns a *borrowed reference* to the referenced
     object. This means that you should always call "Py_INCREF()" on
     the object except when it cannot be destroyed before the last
     usage of the borrowed reference.

PyObject *PyWeakref_GET_OBJECT(PyObject *ref)
    *Valeur de retour : référence empruntée.*

   Similar to "PyWeakref_GetObject()", but does no error checking.

void PyObject_ClearWeakRefs(PyObject *object)
    * Part of the Stable ABI.*

   This function is called by the "tp_dealloc" handler to clear weak
   references.

   This iterates through the weak references for *object* and calls
   callbacks for those references which have one. It returns when all
   callbacks have been attempted.
